The present invention is based on a diode-split high-voltage transformer having a core, a primary winding and a high-voltage winding, which is arranged in chambers of a coil former. The structure of a high-voltage transformer of this type and also the way in which these chambers are wound are explained for example in EP-B-0 529 418.
The high-voltage transformer of a television set or computer monitor is a relatively expensive component, so that it is desirable to simplify its production, but without reducing its operational reliability. The patent application PCT/EP 98/03882, published after the priority date, has already specified a high-voltage transformer in which the high-voltage winding lies below the primary winding, between primary winding and core, whereby this becomes considerably more compact, lighter and more cost-effective. In order to avoid high-voltage flashovers and corona effects, this transformer has an insulation, for example a conductive coating, between the coil former and the core.
It is furthermore desirable for the high-voltage transformer to emit as far as possible no interference radiation, since, due to the high integration level of semiconductor circuits, the chassis of a television set has become very compact in the meantime and irradiation of the tuner circuit is thus possible. In this case, it is particularly diode-split high-voltage transformers that are problematic, since their high-voltage winding is on the outside and has no screening at all, or screening is very complicated and problematic. Measures for reducing this interference radiation or the undesirable oscillations are disclosed for example in EP-A-0 735 552 and EP-A-0 729 160.
The object of the present invention, therefore, is to specify a diode-split high-voltage transformer of the type mentioned in the introduction which is very compact and at the same time has good screening of the interference radiation.
The diode-split high-voltage transformer according to the invention contains a core, a primary winding and a high-voltage winding, the high-voltage winding lying below the primary winding, or within the primary winding with regard to the housing. In this case, the high-voltage winding is arranged in chambers of a coil former whose surface of the inner cavity between the coil former and the core is provided with a conductive coating, thereby avoiding corona effects. Corona effects are produced in particular if a high electric field is present in air or in air inclusions, whereby ozone is produced, which is chemically highly aggressive and destroys the coil former and/or insulation. The conductive coating makes it possible to completely screen the electric field between the high-voltage winding and the core, with the result that no air inclusions or air gaps with high electric fields occur between the conductive coating and the high-voltage winding during operating of the high-voltage transformer.
The conductive coating is advantageously a thin layer containing colloidal graphite. The said layer can be applied in a simple manner by spraying a liquid spraying agent, comprising colloidal graphite and adhesive in a solvent, on the inner wall of the coil former by means of a nozzle. The conductive coating may alternatively be a metallized film which bears tightly on the inner wall of the coil former, or may be formed by potting the interspace between the core and the coil former with a conductive material. Further details concerning the conductive coating are specified in publication No. WO 99/03118, to which reference is hereby made.
The diodes of the high-voltage transformer are situated, in particular, not between or above the chambers with the high-voltage winding, but outside the chambers, with the result that the primary winding, taking a corresponding insulating layer into account, can be arranged directly above the chambers and is tightly wound in such a way that the high-voltage winding is completely covered by the primary winding. As a result of this, together with the conductive coating on the underside of the coil former, outstanding screening is produced for the high-voltage winding. It is appropriate, moreover, at least in the case of high-voltage transformers having two and four diodes, to connect one outer chamber to earth and to provide the other outer chamber as a high-voltage connection, with the result that the high-voltage transformer is also completely screened laterally, and to the top and bottom in the case of an upright design.
For the screening effect of the conductive coating, the latter should be earthed or connected to a constant electrical potential. It has been shown, however, that the thin electrical coating cannot be contact-connected to a metallic conductor without problems, since the said conductor can only be clamped on and not soldered, and the conductor only enables contact at points, or only a very small surface of the conductive coating is contact-connected. Since the conductive coating has, in particular, a high impedance in order to avoid eddy currents, the contact point to the earth connection can be destroyed by compensating currents. A measurement of the resistance across the conductive coating in the length of the coil former yields resistances of between 20 kohms and 2 Mohms, for example, depending on the design.
This earth connection can be avoided, however, if the chambers are arranged and wired to the diodes in such a way that the oscillations which arise during operation of the diode-split high-voltage transformer, in particular in the blocking phase of the diodes, induce capacitive currents on the conductive coating, which currents mutually cancel one another out, in other words the sum of these currents is zero. This can be achieved for example in that in the chambers the interference oscillations occur with identical amplitudes but in antiphase, and the capacitances between the chambers and the conductive coating are identical, with the result that the capacitive currents on the conductive coating compensate for one another. It is preferable to use an even number of chambers which all have an identical number of turns or at least an identical number of turns in pairs, with the result that the oscillations occur with quantized amplitudes. By virtue of the connections of the chambers to one another and to the diodes, oscillations with rising and falling amplitudes occur in one direction, with the result that the compensating currents of two respective chambers whose oscillations have the same amplitudes compensate for one another on the conductive coating.
In this case, a group of chambers in the centre of the high-voltage transformer has between two chambers a pulse-free connection which can advantageously be used for the focus connection of a picture tube. When the chambers are being wound, it is necessary in this case to ensure that chambers that are not yet filled are not straddled by wires, and the winding sense of the chambers is uniform.
Since the compensating currents cancel one another out, the interference radiation of the oscillations arising in the high-voltage winding is effectively screened, even if the earth connection for the conductive coating is omitted. The chamber bottoms have, in particular, the same thickness, for example 1 mm, with the result that the capacitances produced between the chambers and the conductive coating are identical. Final zero balancing of the output currents may furthermore be effected by different numbers of turns in individual chambers, whereby it is possible to reduce remaining pulse voltages from, for example, 40 V down to approximately 0 V. For monitoring purposes, it is possible in this case to measure the compensating current between the conductive coating and a reference-earth potential, for example earth. In the event of ideal balancing, the said current decreases to zero.
In the case of a high-voltage transformer having two diodes, the chambers with the high-voltage winding are subdivided into three groups by the two diodes, the highest pulse voltages occurring on both sides across the two diodes and the focus connection being routed out from the middle chamber and being free of pulse voltage.
In the case of high-voltage transformers having three and four diodes, a corresponding arrangement and wiring or winding of the chambers likewise make it possible to achieve the result that the capacitive currents on the conductive coating compensate for one another, with the. result that an earth connection can also be avoided in the case of these. In this case, the chambers are likewise preferably designed in such a way that oscillations occur with the same amplitude but in antiphase. These also contain a middle group with an even number of chambers, so that a focus voltage which is free of AC voltage can be routed out.
The present high-voltage transformer is thus excellently suited to recent television sets or monitor chassis since it operates practically with no interference radiation. It need no longer be feared that interference radiation will interfere with the tuner circuits. Contact-connection of the conductive coating, which is complicated with a reliable design and thereby increases the cost of the high-voltage transformer, can be avoided.